Visual attention is controlled in two distinct, well-known systems: the top-down selection system and the bottom-up selection system. Though numerous studies have addressed these two selection systems, the difference in neuronal mechanism between them is still unclear. Especially, because the bottom-up selection system normally requires less effort and induces less brain activity than the top-down selection system does, it is difficult to detect the neural mechanism for the bottom-up system compared to that for the top-down system.

In the present study, we used a visual search task, involving two conditions: a feature search condition that would induce top-down selection and a singleton search condition that would induce bottom-up selection, first introduced by Bacon and Egeth (1994). In the first psychological experiment, the task-difficulty and the search-efficiency in each condition were matched to equate the amount of effort required for each condition. Subsequently, we measured brain activity for the two conditions. As a result, in addition to extended activation in frontal and parietal areas: bilateral FEF, bilateral ventral premotor areas, ACC, bilateral parietal areas, and bilateral visual cortices, for both conditions, a direct comparison between the singleton search condition and the feature search condition revealed that the area around the bilateral intraparietal sulci were more involved in the singleton search mode.